A broadband access may be implemented by a passive fiber optical access network, e.g. by a B-PON (Broadband Passive Optical Network or a G-PON (Gigabit-capable Passive Optical Network). A passive optical network comprises an OLT (Optical Line Terminal) and one or more ONTs (Optical Network Terminations) and/or ONUs (Optical Network Units) connected by an ODN (Optical Distribution Network), which includes optical fibers and passive optical power splitters, since a PON does not require active electrical components for splitting the optical signals.
The OLT is located at a Central Office, CO, associated with a service provider (e.g. an operator), and the CO provides an interface for delivery of services, e.g. telephony, Ethernet data or video, to subscribers/end-users over the PON. Each ONT/ONU terminates the PON and converts the optical signals into electrical signals for delivery of the services to terminals of the subscribers/end-users, via a suitable user interface.
Typically, a single optical fiber of the PON carries the traffic from the OLT to a Remote Node, RN, in which the traffic signal may be split by passive optical splitters into a number of separate optical fibers (drop links), each one being terminated by an ONU or an ONT, in an optical point-to-multipoint link. However, the traffic signal from the OLT could also be connected to only one ONT/ONU in a point-to-point link.
A so-called high-loss budget PON may involve an OLT that is connected to a comparatively large number of ONTs/ONUs via a point-to-multipoint link, which comprises a Remote Node with a high split ratio, typically larger than 1:32.
The above-mentioned terms ONU and ONT both indicate a device that terminates any one of the distributed (leaf) endpoints of an ODN, implements a PON protocol, and adapts PON PDU (Protocol Data Units) to a subscriber-service interface. The term ONT may typically indicate a single subscriber device, e.g. in a home, and the term ONU may indicate a device that could be provided with additional functionality.
However, the term ONT will hereinafter be used as a generic term in this disclosure and in the appended claims, and will refer to any of an ONU or an ONT.
Since faults may occur in a PON, e.g. due to break in an optical fiber or due to a deteriorating or non-functioning OLT or ONT, a PON has to be monitored, and failures detected and located.
The optical fibers and the optical components of the PON may be characterized e.g. by the use of Optical Time Domain Reflectometry, OTDR. When characteristics of an optical fiber are determined using a conventional OTDR, a series of optical pulses are injected into the fiber. The optical pulses, i.e. an OTDR signal, travels through the fiber, and the OTDR signal is reflected and received, e.g. by the device that injected the optical pulse into the fiber. The amplitude of the reflected, or backscattered, OTDR signal is measured, and may be used e.g. for estimating the length and the attenuation of the fiber, or for locating faults, e.g. a break in the fiber.
A monitoring of a passive optical network using OTM (Optical Transceiver Monitoring) and the above-described OTDR, with a detection and localization of faults with high sensitivity and without requiring upgrading of the ONT, is described e.g. in Urban et. al: “WDM-PON Fiber-Fault Automatic Detection and Localization with 1 dB Event Sensitivity in Drop Links” Conference paper, National Fiber Optic Engineers Conference (NFOEC), Mar. 4, 2012.
The OTDR may provide an accurate localization of a failure in a point-to-point link. However, the dynamic range of OTDR is limited to around 40 dB, and misinterpretations may arise when one of the drop links after a Remote Node with a high split ratio is faulty, since the received backscattered signal is a superposition of the optical power returning from all drop links.
In order to provide a localization of faults in a point-to-multipoint link, the OTDR may e.g. be assisted by a reflective component installed in each drop link, wherein a decrease of the reflection peak in the reflectogram will point out the faulty drop link. However, an introduction of a new component in an ONT will increase the complexity and the costs, e.g. since the equipment is located at customer premises.
Thus, it is desirable to enable an improved monitoring of a passive optical network, also of a point-to-multipoint link with a high split ratio, in order to detect and locate faults in an optical fibre, in an OLT at the Central Office, or in an ONT at customer premises, in a short time frame and in a cost-efficient way.